Pipe inspection is important in the maintenance of pipe systems such as subterranean waste water drain systems. Traditionally, pipe inspection is accomplished using a camera head attached to the distal end of a semi-rigid push-cable wrapped in turns inside a rotatable drum or reel. The camera head is inserted into the pipe to be inspected and the push-cable is paid out to force the camera head down the pipe. The camera in the camera head sends back still and/or video image signals over conductors embedded in the push-cable. Images of the interior of the pipe generated from the signals are viewed in real time and recorded for later study.
One of the difficulties in pipe inspection is the accurate measurement of the distance the camera head has traveled into the pipe. This information is important in precisely locating blockages or breaks in the pipe. Various techniques have been developed for measuring the distance that a camera head has been pushed down a pipe via a push-cable. For example, U.S. Pat. No. 6,545,704, granted Apr. 8, 2003 to Mark S. Olsson et al., discloses a video pipe inspection system that uses a distance sensing module including two non-contact sensor pairs. U.S. Pat. No. 6,505,525, granted Jan. 14, 2003 to R. Michael McGrew, discloses a sewer pipe inspection system in which a rotor magnet is rotated as a push rod cable is inserted into a pipe. A pair of Hall effect switches are mounted adjacent the rotor magnet and a decoder circuit connected to these switches is used to generate a linear distance traveled by the push rod cable and a footage signal is displayed to the user.